The effect of the octan-3-yloxy and the octan-2-yloxy chiral moieties on the mesomorphic properties of ferroelectric liquid crystals

Article abstract of DOI:10.1039/c5ra14903g

New mesogenic compounds exhibiting unique, so called orthoconic, behavior at the synclinic smectic SmC? phase have been obtained. The newly synthesized compounds belong to two chiral homologous series: 4′-[ω-(2,2,3,3,4,4,4-heptafluorobutoxy)alkoxy]bipheny

Full text of DOI:10.1039/c5ra14903g

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The effect of the octan-3-yloxy and the octan-2-
yloxy chiral moieties on the mesomorphic
properties of ferroelectric liquid crystals†
Cite this: RSC Adv., 2015, 5, 81003
*
Dorota Wegłowska, Paweł Perkowski, Wiktor Piecek, Mateusz Mrukiewicz
˛
and Roman Dabrowski
˛
New mesogenic compounds exhibiting unique, so called orthoconic, behavior at the synclinic smectic SmC*
phase have been obtained. The newly synthesized compounds belong to two chiral homologous series: 40-
[u-(2,2,3,3,4,4,4-heptafluorobutoxy)alkoxy]biphenyl-4-yl-4-(octan-2-yloxy)benzoates and 40-[u-(butoxy)
alkoxy]biphenyl-4-yl-4-(octan-3-yloxy)benzoates. Their mesogenic behavior has been studied and their
phase transition temperatures as well as enthalpies have been evaluated using polarizing optical
microscopy, differential scanning calorimetry and dielectric spectroscopy techniques. The tilt angle, the
spontaneous polarisation as well as the helical pitch of the compounds have been studied in the full
temperature domain. The compounds with a 40-u-(2,2,3,3,4,4,4-heptafluorobutoxy)alkoxy terminal chain
exhibit a polar smectic C* phase. The analogous compound with a 40-u-(butoxy)alkoxy achiral terminal
chain and an octan-2-yloxy chiral part exhibits an Iso-N*-SmC* phase sequence, while the one with an
octan-3-yloxy chiral part does not exhibit mesogenic behavior. The compounds with the octan-3-yloxy
chiral part have much lower melting points than those with the octan-2-yloxy chiral part. The clearing
points of the new compounds decrease with the increase of the length of the oligomethylene spacer
chain. Dielectric studies confirmed the presence of SmC* and N* phases. The tilt angles measured in the
SmC* phase reveal extremely high values at saturation approaching 45ꢀ. The values of the spontaneous
polarization for all investigated compounds are as high as 89.5 nC cmꢁ2. The length of the helical pitch for
different compounds varies from 460.7 nm to 1367.7 nm.
Received 27th July 2015
Accepted 9th September 2015
DOI: 10.1039/c5ra14903g
www.rsc.org/advances
V-shaped switching was developed by the Japanese company
Dainippon Ink and Chemicals. FLCs working in the PSV mode
1 Introduction
exhibit short switching times (100–200 ms (ref. 12)) and high
contrast ratios. In addition, they require a low applied voltage
(<10 V) and the phase sequence: Iso-N*-SmA*-SmC* (INAC
phase sequence). In the PSV effect the length of the helical pitch
of the FLC plays no role.
In the deformed helix ferroelectric (DHF) effect13–16,24 the
helix axis is parallel to the substrate plane. This electro-optical
effect is observed while the helical structure is affected by a
weak electric eld E (which is less than the critical eld EC of the
helix unwinding). The critical electric eld EC and the switching
time s are given by eqn(1) and (2):
Since N. A. Clark and S. T. Lagerwall demonstrated the fast
switching electro-optical effects based on surface stabilized
ferroelectric liquid crystals (SSFLCs) in 1980 (ref. 1), ferroelec-
tric smectic liquid crystals (FLCs) have been extensively
studied.2–9 The main problem with the use of FLCs in photonic
applications is the difficulty in obtaining homogenous and
permanent structures (resistant to mechanical shock) of the
smectic layers inside the liquid crystal cell. The mechanical
shock resistance of FLCs can be increased by stabilizing the
smectic layer with a polymer network.10
In 1995 Fukuda mentioned for the rst time the V-shaped
electro-optic response11 exhibiting electro-optical characteris-
tics similar to that observed for nematic liquid crystals, which
has potential to become a very promising technology in
photonic applications. The electro-optical effect in ferroelectric
liquid crystals stabilized by polymer networks (PSV) exhibiting
g4p2
K4p2
s ¼
(1)
and
p4
K
EC
¼
4 PsP2
(2)
Faculty of Advanced Technologies and Chemistry, Military University of Technology, 2
Kaliskiego Str., 00-908 Warsaw 49, Poland. E-mail: dorota.weglowska@wat.edu.pl
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra14903g
where: g4 is the rotation viscosity, p is the helical pitch, K is the
elastic constant and Ps is the spontaneous polarization.
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The electric eld value required for the helix deformation
and the deviation of the optical axis of the FLC slab in the plane
of the measuring cell is low.22
Da # q
(6)
The helical conguration in the DHF mode exhibits a wide
range of unique optical properties such as circularly
The helical pitch p of the FLC structure within a cell prepared
for the observation of the DHF effect is much smaller than the
cell gap d (p ꢂ d), so that the structure can be considered to be
free from boundary surface induced distortions.17 In conse-
quence, the helix becomes wound within the cell18 in contrast to
the SSFLC effect, where the surface induces unwinding of the
helix and it requires the helical pitch of the FLC to be longer
than the cell thickness. The director of the ferroelectric liquid
crystal structure inside the cell in the DHF mode is arranged in
a similar way as in SSFLC effect, but with such a difference that
the helix of the FLC is not unwound. The DHF mode has been
regarded as very promising for display and photonic applica-
tions. When an electric eld is applied that is lower than the
critical electric EC eld between the two substrate plates it
couples to the spontaneous polarization in each molecular
layer. The helical structure becomes deformed and then fully
untwisted for an electric eld higher than the critical eld EC.
The helical pitch for the DHF effect should be preferably short
(p < 1 mm) and the tilt angle should be relatively large (q >
30ꢀ).18,19 In the DHF mode, the light transmission T of the cells,
placed between crossed polarizers (see Fig. 1) is described by
relationship (3):20
polarized Bragg type reections and
rotatory dispersion. The switching time in the DHF mode
is short, less than 100 ms, at very low applied
a huge optical
a
voltage (1 V mmꢁ1). The switching curve is a hysteresis-
free V-shape and is nearly independent of the frequency of
the applied voltage in a broad frequency range (10 Hz to 4
kHz).
The above features make FLCs working in the DHF mode
very useful for tunable lters, thermography, ele-
ctrically tunable optical diodes,23 biosensors,24 voltage
sensors,25–28 spatial light modulators,29–32 real-time multi-
point measurements, under water sonar array systems33–35
such as ber optic hydrophone array systems that could be
used for underwater acoustic surveillance applications
(e.g. military, counter terrorist and customs authorities for
protecting ports and harbours) and many other various
applications.36
For the above mentioned electro-optic effect ferroelectric
liquid crystals having a low melting point, a broad temper-
ature range of SmC* phase, a high tilt angle, high sponta-
neous polarization and a short helical pitch are especially
promising. The assortment of high tilted FLCs is still
limited.
pdDneff
T ¼ sin2 2½b ꢃ Daꢄsin2
(3)
l
There is a strong relationship between the molecular
structure of the FLCs and their mesomorphic and electro-
optical properties.37,38 Even a small change of the mole-
cular structure inuences its properties.39–41 The rst family of
highly tilted FLCs was described by T. Inukai et al.42 (see I; The
black dots conrm the presence of the particular phase):
where: b is the angle between the polarizer and the helix axis x
of the ferroelectric phase; Da is the shi of the helical axis due
to the electric eld; Dneff is an effective birefringence and l is
the wavelength.
Regardless of the polarity of the driving voltage an electro-
optical response is obtained for the geometry with b ¼ 0.21
The maximum light transmission under this condition occurs
if:
Da ¼ p/4
(4)
and
q
[ꢀ]
pdDneff
¼ p=2
(5)
n
Cr
SmC*
N*
Iso
l
7
8
9
C
C
C
88.2
78.3
79.1
C
C
C
93.5
98.4
101.0
C
C
C
120.4
121.1
117.1
C
C
C
45
45
45
so, the tilt angle q of the FLC should be close to 45ꢀ (p/4) to
provide the maximum light transmission at b ¼ 0:20,22
In compounds I the molecular director is tilted with respect
to the smectic layer’s normal at an angle of 45ꢀ. They exhibit a
phase sequence: Iso-N*-SmC*. With the elongation of the
alkoxy chain CnH2n+1O– the stability of the SmC* phase
increases and the melting point of the compounds decreases.
Compounds I synthesized by Inukai exhibit high melting
points, which makes such compounds less useful for photonic
applications.
In 2011 C. Liao et al.43 synthesized compounds with a
siloxane terminal chain exhibiting much lower melting points
(compounds II):
Fig. 1 A schematic drawing of the DHF mode in a planar cell.
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(ꢁ)-octan-2-ol] are presented in the ESI.† The preparative
procedures of the required chiral phenol 8b [(S)-(+)-40-
hydroxybiphenyl-4-yl-4-(octan-2-yloxy)benzoate] has been pre-
sented in our earlier papers.44,45 For the chiral functionalization
(compound 4a) a Mitsunobu etherication47 has been used.
During this reaction the conguration at the chiral carbon atom
is inverted. This method was also used to couple the chiral
q
[ꢀ]
s
[ms]
m
n
Cr
SmC*
Iso
6
11
2
3
C
C
1.3
46.9
C
C
88.9
106.9
C
C
45
44
23
35
Switching times of such compounds are very short (less than
40 ms), but the applied voltage is high (160 V at the cell gap of 7.5
mm). Such behaviour is ascribed to the presence of the bulky
dimethylsiloxane group, which shows less exibility and a
smaller number of conformational states than the alkyl chain
compounds.
Continuing our previous studies44,45 we have decided to
synthesize two homologous series of new chiral esters with
general structures III:
where: X ¼ H or F; m ¼ 2–7; either: R1 ¼ CH3 and R2 ¼ C6H13 for
(S)-(+)-octan-2-yloxy derivatives or R1 ¼ C2H5 and R2 ¼ C5H11 for
(R)-(ꢁ)-octan-3-yloxy derivatives.
These compounds are abbreviated as 3XOmCk, where X is
the hydrogen or uorine atom at the achiral terminal chain, m is
the length of the oligomethylene spacer between the rigid core
and the rst alkoxy group of the terminal chain and k is the
number of carbon atoms in the R1 group at the chiral terminal
chain. Either R1 is a methyl group (k ¼ 1) and R2 is an hexyl-
group [obtained from (S)-(+)-octan-2-ol; series nXOmC1] or R1
is an ethyl group (k ¼ 2) and R2 is a pentyl group [obtained from
(R)-(ꢁ)-octan-3-ol; series nXOmC2].
Temperatures and enthalpies of the phase transitions, tilt
angles, spontaneous polarizations as well as the helical pitch in
the SmC* phase of homologues described above have been
examined.
2 Experimental
2.1 Synthesis
The method of synthesis of the nal compounds 3XOmC2
[prepared from (S)-(+)-octan-3-ol] is shown in Scheme 1 and
described in a MSc thesis.46 The preparative procedures of the
Scheme 1 The synthesis route of compounds 3XOmC2 (prepared
nal compounds 3XOmC2 and 3XOmC1 [prepared from (R)- from (S)-(+)-octan-3-ol).
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phenol (8a) with an appropriate butoxyalkanol or 2,2,3,3,4,4,4- analogues the nematic phase above and a more ordered tilted
heptauorobutoxy alkanol. The method of the synthesis of smectic phase below the synclinic SmC* phase have been
2,2,3,3,4,4,4-heptauorobutoxy alkanols is described by P. Kula observed upon cooling in compound 3HO3C1. Similar to the
et al.48
Inukai compounds with the alkyl or alkoxy terminal chain the
The preparative procedures of the nal compounds 3XOmCk Iso-N*-SmC* phase sequence has here been observed. The
1
and their characterization by GC-MS, HPLC-MS and by H and melting points of the compounds of series 3FOmC1 decrease
13C NMR spectroscopy are presented in the ESI.†
with the increase of the index m in the terminal achiral alkyl
chain. An exception is compound 3FO7C1, which exhibits a
slightly higher melting point than compound 3FO6C1.
Members 3FO3C1, 3FO5C1 and 3FO7C1 exhibit lower clearing
points than members 3FO2C1, 3FO4C1 and 3FO6C1, so the
characteristic odd-even effect is observed.49 In Fig. 2 the effect
of the number of carbon atoms m in the oligomethylene
spacer on the melting and clearing points for the compounds
3FOmC1 is presented.
The uorinated members of the homologous series
3FOmC2, wherein m ¼ 2, 5 and 7 exhibit the enantiotropic
SmC* phase accompanied with the direct SmC*-Iso transition,
while the member with m ¼ 3 (compound 3FO3C2) exhibits the
monotropic SmC* phase. Their clearing and melting points are
much lower. This is a typical behavior observed for compounds
with a larger branched chain (an ethyl instead of a methyl
group). The melting points of compounds of the series 3FOmC2
decrease with the increase of the length of the oligomethylene
spacer (index m). Compound 3FO7C2 exhibits the lowest
melting point, only 36.8 ꢀC. For the protonated compound
3HO3C2 (no uorine atom substituted) no mesophase is
observed.
3 Results and discussion
3.1 Mesomorphic properties
Mesophases have been identied conventionally by observing
textures using an Olympus polarizing optical microscope (POM)
with crossed polarizers and equipped with a Linkam TMSH 600
hot stage and a Linkam TMS 93 temperature controller. Phases
have been additionally conrmed on the basis of the results of
the dielectric studies. The temperatures and enthalpies of the
phase transition have been determined using differential
scanning calorimetry with a DSC SETARAM 141 instrument
with the scanning rate 2 ꢀC minꢁ1 in both heating and cooling
cycles. In Table 1 the temperatures and the enthalpies of the
phase transitions of compounds from series 3XOmCk are
presented.
Compounds of the uorinated series 3FOmC1 and
3FOmC2 exhibit the SmC* phase only (with the direct SmC*-
Iso transition). The members of the homologous series
3FOmC1 with m ¼ 5, 6 and 7 exhibit very high melting
enthalpies (above 32 kJ molꢁ1). In the case of the protonated
Table 1 The phase transition temperatures [ꢀC] (upper row; onset point) and the enthalpies [kJ molꢁ1] (lower row) of the members of the
homologous series 3XOmCk from DSC measurements determined during heating (values given in brackets were determined upon cooling)a
Acr.
Cr2
Cr
C
C
C
C
C
C
C
C
C
C
C
C
SmX*
SmC*
C
N*
Iso
C
C
C
C
C
C
C
C
C
C
C
C
3FO2C1
3FO3C1
3FO4C1
3FO5C1
3FO6C1
3FO7C1
3HO3C1
3FO2C2
3FO3C2
3FO5C2
3FO7C2
3HO3C2
96.0
27.96
84.3
25.93
82.2
14.98
77.8
37.01
70.8
32.79
74.5
33.13
74.3
39.89
87.0
24.83
73.7
38.78
49.6
27.10
36.8
16.44
40.1
132.4
8.62
114.5
6.58
135.6
8.53
123.7
7.03
129.2
8.21
123.1
7.71
(71.2)
ꢁ3.83
92.6
C
43.1
3.91
C
C
C
C
C
37.3
8.93
C
C
(55.7)
ꢁ0.71
C
C
83.1
0.77
C
5.40
C
(72.4)
ꢁ3.75
75.6
3.32
78.0
C
C
3.55
33.97
a
Acr. stands for acronym.
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at a low (0.1 V) applied measuring voltage. Such an AC voltage is
enough for the electric current response to be easily interpreted
by the impedance analyzer. Additionally, such AC voltages
create an electric eld (E) which is far from the level producing
nonlinear effects in liquid crystals. A 0.1 V electric voltage of the
AC measuring signal allows us to ignore nonlinearity in the
dielectric response. During measurements no bias (DC) voltage
was applied. Measuring frequencies varied from 100 Hz up to 10
MHz. The temperature of the measuring cells was controlled
using a computer driven Linkam TMS 92 unit and a hot stage
ꢀ
Linkam THMSE 600 at an accuracy of 0.1 C. During ꢁm1easure-
ꢀ
ments cells were slowly cooled at a rate of 0.3 C min
.
In Fig. 4 the real part 30 of the electric permittivity at six
frequencies (0.1, 1, 10, 100 kHz, 1 MHz and 10 MHz) versus the
temperature for compound 3FO5C2, chosen as a typical
member of the uorinated series, is shown. At frequencies
below 1 kHz a strong dispersion is observed. This dispersion is
typical for the SmC* phase. It falls with the increase of the
frequency. The observed mode can be interpreted as a Gold-
stone mode.52 The Goldstone mode is a collective relaxation,
which is related with the procession of tilted molecules around
the helical axis observed in the ferroelectric SmC* phase. It is
the strongest mode observed in liquid crystals built from rod-
like molecules. It is not an Arrhenius-type relaxation. The
relaxation frequency of the Goldstone mode varies usually from
100 Hz to 10 kHz. Using our calculation procedure53,54 the
parameters of the Goldstone mode for four temperatures were
calculated. The results are shown in Table 2. The dielectric
strength d3 for compound 3FO5C2 decreases with the temper-
ature decrease. The same effect is observed for the relaxation
frequency of the Goldstone mode. Due to the fact that in the
investigated compound a direct nucleation of clusters in the
SmC* phase was observed at the temperature of the phase
transition from the isotropic liquid one cannot observe the so
mode.52 One can see that the dielectric spectroscopy results
conrm the phase sequences observed from the differential
scanning calorimetry (DSC) measurements. Transitions
Fig. 2 The effect of the number of carbon atoms in the oligo-
methylene spacer on the melting points (the Cr-SmC* transition) and
clearing points (the SmC*-Iso transition) for compounds 3FOmC1.
Compounds of the series 3FOmC1 exhibit the focal-conic
texture of the SmC* phase, while members of the series
3FOmC2 form strongly defected microscopic patterns of the
SmC* phase with small domains, see Fig. 3.
3.2 Dielectric measurements
The measurements have been taken using a HP 4192A imped-
ance analyzer and custom-made measuring cells.50 A low-
resistivity ITO layer (10 U sqꢁ1) were used to avoid the high-
frequency losses related to the nite conductivity of the ITO
electrodes.51 Short wires of a low-resistivity were used for the
connection of the measuring cells to the impedance analyzer.
Cells with a thickness of 5 mm covered with polyimide SE130 as
a
homogeneous aligning layer and anti-parallel rubbed
(wherein the glass substrates were rubbed in the opposite
directions) were used. Due to the cell gap being small, applying
a small value of AC voltage (0.5 V) creates a large electric eld
E ¼ 100 kV mꢁ1. For this reason measurements were conducted
Fig. 3 Micrographs of the textures observed during cooling for Fig. 4 The real part 30 of the electric permittivity as a function of
compounds 3FO6C1: (a) during the Iso-SmC* transition at 129.0 ꢀC, temperature (T) for compound 3FO5C2 measured during cooling for
(b) the SmC* phase at 90.2 ꢀC and 3FO5C2: (c) during the Iso-SmC* 100 Hz, 1, 10, 100 kHz, 1 MHz and 10 MHz. Arrows indicate the phase
transition at 75.5 ꢀC and (d) the SmC* phase at 53.7 ꢀC.
transitions.
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Table 2 The parameters of the Goldstone mode (dielectric strength
d3, and the relaxation frequency fR) for compounds 3FO5C2 and
3HO3C1
The relaxation frequency of the Goldstone mode of compound
3HO3C1 is higher than in the case of compound 3FO5C2,
because the 3FO5C2 molecule is much larger and heavier than
the 3HO3C1 molecule. Longer and larger objects show lower
relaxation frequencies than shorter and lighter ones.
Acronym Parameter 70 ꢀ
C
60 ꢀ
C
50 ꢀ
C
40 ꢀ
C
3FO5C2
3HO3C1
d3
fR
d3
fR
52.2
420 kHz
12.4
44.2
340 kHz
11.2
40.6
28.5
In the case of compound 3FO5C2 in the isotropic phase, near
to the Iso-SmC* transition, a weak dispersion is observed at
high frequencies. It is worth noting that this can be interpreted
as the dielectric response of the molecular rotation around a
short molecular axis (S-mode).55 This S-mode is strongly
temperature dependent. In the SmC* phase the S-mode is
hindered. Such a mode should be easily detectable both in the
isotropic and in the nematic phases, but the molecules of
compound 3HO3C1 are too light in comparison with the
molecules of compound 3FO5C2 and hence the relaxation
frequency of the S-mode in compound 3HO3C1 is much higher
than in compound 3FO5C2 and cannot be observed using our
experimental setup (limited frequency measuring range).
220 kHz 150 kHz
2900 kHz 2020 kHz
observed during the cooling cycle for compound 3FO5C2 are as
follows: Iso 77.5 ꢀC SmC* 31.5 ꢀC Cr. The plot for the frequency
of 10 MHz is inuenced by cell properties53,54 due to resistivity of
the ITO electrodes. This means that the 30 values presented for
10 MHz are slightly underestimated.
In Fig. 5 the real part 30 of the electric permittivity at six
frequencies (0.1, 1, 10, 100 kHz, 1 MHz and 10 MHz) versus the
temperature for compound 3HO3C1 is shown. The plot for 10
MHz is inuenced by the properties of the measuring cell (cut-
off frequency of the measuring cell), as it is for the measure-
ments of 3FO5C2. The observed dielectric spectrum conrms all
phases observed using the DSC method. The phase transition
temperatures determined using dielectric spectroscopy differ a
little in comparison with the DSC results: Iso 84.5 ꢀC N* 72.5 ꢀC
SmC* 55 ꢀC SmX 48.5 ꢀC Cr. In addition, a more ordered
smectic phase (SmX) with hindered dispersion below the SmC*
phase is observed. The well dened Goldstone mode is observed
too, while the so mode is not observed in the dielectric
response of 3HO3C1, due to the absence of the SmA* phase. The
parameters of the Goldstone mode for two values of the
temperature: 70 and 60 ꢀC were calculated. The results of these
calculations are presented in Table 2.
3.3 Tilt angle measurements
The tilt angle was studied using the standard method56 with
custom made cells with a bookshelf-like structure of the liquid
crystal in the SmC* phase. Cells with a thickness of 1.5 mm with
ITO electrodes were coated with polyamide nylon 6.6 to obtain a
homogeneous alignment layer. Aligning layers were deposited
from a 0.5% solution of the dry mass in 3-uoroethanol by
spinning. Aer the drying and baking processes cell substrates
with polyamide orienting layers were rubbed unidirectionally
and assembled using a rod-like spacer in the sealing frame only.
Cells were lled with the compound to be investigated using
capillary action at a temperature elevated enough to keep the
material under study in the isotropic phase. A homogenous
texture with the planar alignment of the director at the smectic
layers was obtained upon slow (0.01 ꢀC minꢁ1) cooling. The tilt
angle q in the temperature domain was observed during the
The dielectric strength d3 exhibited by compound 3HO3C1 is
four times smaller than in the case of compound 3FO5C2 (at the
same value of temperature). It means that uorine atoms can
support a stronger dielectric response of the Goldstone mode.
cooling cycle in the presence of an electric eld (E ¼ 2.8 V mmꢁ1
;
f ¼ 15 Hz) of magnitude enough to saturate the switching angle.
The transmitted light intensity on the angle between the
direction of the smectic layer normal and the orientation of the
polarizer for both electric eld polarizations was observed. The
tilt angle was obtained as half of the angle between the two
minima of the light transmissions for both of the polarizations
of the electric eld applied.
In Fig. 6 the values of the tilt angles q as a function of temper-
ature for compounds 3FO6C1, 3FO7C2 and 3HO3C1 measured in
the SmC* phase are presented. A maximum optical tilt angle of
44.9ꢀ was observed at 106 ꢀC for compound 3FO6C1, 45.0ꢀ at 40 ꢀC
for compound 3FO7C2 and 42.9ꢀ at 61 ꢀC for compound 3HO3C1.
The values of the tilt angle in all the investigated compounds
depend on the temperature slightly at the same distance from the
Iso-SmC* transition and increase with the temperature decrease.
3.4 Spontaneous polarization measurements
Fig. 5 The real part 30 of the dielectric permittivity as a function of
temperature for compound 3HO3C1 measured during cooling for 100
Hz, 1, 10, 100 kHz, 1 MHz and 10 MHz. Arrows indicate the phase
transitions.
The values of the spontaneous polarization (PS) were studied
using the reverse current method57,58 with a triangle pulse of the
AC electric eld. Cells with a cell gap of 3.0 mm were prepared in
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3.5 Helical pitch measurements
The helical pitch measurements were carried out based on the
selective light reection phenomenon.59,60 The wavelength of
selectively reected light was calculated by the observation of the
minimum of the light transmission measured using a UV-VIS-
NIR 3600 Shimadzu spectrophotometer in the range of 360–
3000 nm during the cooling cycle. Cells were cooled from the
isotropic phase and the temperature was controlled using the
Linkam S-1700 temperature controller. The maximum of the
selective reection of the light in the SmC* phase for compounds
3FO3C1, 3FO6C1, 3HO3C1, 3FO3C2, 3FO5C2 and 3FO7C2 was
measured. The helical pitch in the measured compounds was
ˇ
calculated using the equation: lmax ¼ np, where lmax is the length
ˇ
of the selectively reected light and n is the average refractive
Fig. 6 The tilt angle q as a function of temperature for compounds
3FO6C1, 3FO7C2 and 3HO3C1 measured during cooling.
ˇ
index (for the investigated compound n is about 1.5 (ref. 61)). The
glass plate, covered by a thin layer of the investigated compound,
was placed into the spectrophotometer in the way of the light ray
at the side. The selective reection was registered every 1 ꢀC. The
calculated lengths of the helical pitches for the measured
compounds are shown in Fig. 8.
a similar way as ꢀfor the tilt angle measurements. The samples
were slowly (0.1 C minꢁ1) cooled from the isotropic phase in
the presence of an electric eld (E ¼ 2.8V mmꢁ1; f ¼ 15 Hz).
Values of the PS were obtained from the integration of the
polarization reversal current peaks registered in response to the
application of the triangular shaped voltage pulse.
The length of the helical pitch in compounds 3FO3C1 and
3FO6C1 in the SmC* phase decreases with the temperature
decrease from 510.7 nm at 107 ꢀC to 460.7 nm at 71 ꢀC and from
1009 nm at 105 ꢀC to 927 nm at 70 ꢀC respectively. For compounds
3HO3C1, 3FO3C2, 3FO5C2 and 3FO7C2 the length of the helical
pitch decreases with the temperature increase: from 1024.7 nm at
74 ꢀC to 840 nm at 77 ꢀC, from 693.3 nm at 71 ꢀC to 720 nm at 65
ꢀC, from 1096.7 nm at 44 ꢀC to 902 nm at 71 ꢀC and from 1367.7
nm at 35 ꢀC to 938 nm at 80 ꢀC respectively. For compound
3HO3C1 the wavelength of the selective reection in the SmC*
In Fig. 7 the results of the polarization measurements at
different temperatures for the SmC* phase for compounds
3FO6C1, 3FO7C2 and 3HO3C1 are shown. The values of the
spontaneous polarization for the investigated compounds are
relatively high: 89.5 nC cmꢁ2 at 101 C for compound 3FO6C1,
ꢀ
91.4 nC cmꢁ2 at 40 ꢀC for compound 3FO7C2 and 103.1 nC cmꢁ2
ꢀ
at 59 C for compound 3HO3C1. The values of the spontaneous
ꢀ
polarization for all investigated compounds increase with the
temperature decrease. The compound with a butoxypropoxy
achiral chain (3HO3C1) has the highest value of spontaneous
polarization. The same relation was observed in the recently
investigated benzoates with alkanoyloxyalkoxy and per-
uoroalkanoyloxyalkoxy achiral chains.43,44 For 4-octyl-2-
oxycarbonylobiphenylyl-4-yl-4-(alkanoyloxy-alkoxy) benzoates and
peruoroalkanoyloxyalkoxy benzoates the opposite relation was
observed. The uorinated compounds exhibit higher PS values.
phase below 74 C is out of the range of the spectrometer. The
helical pitch p depends on the length of the oligomethylene
spacer and decreases with the decrease of the index m.
4 Discussion and conclusions
The prepared compounds with the uorinated 2,2,3,3,4,4,4-
heptauorobutoxy unit in the achiral chain of both homologous
Fig. 8 The length of the helical pitch vs. temperature calculated for
Fig. 7 Spontaneous polarization PS as a function of temperature for compounds 3FO3C1, 3FO6C1, 3HO3C1, 3FO3C2, 3FO5C2 and
compounds 3FO6C1, 3FO7C2 and 3HO3C1 measured during cooling. 3FO7C2 in the SmC* phase measured during cooling.
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series 3FOmC1 and 3FOmC2 exhibit the Cr-SmC*-Iso phase
sequence and their clearing points decrease with the increase of
the m index in the oligomethylene spacer of the terminal achiral
chain. The compound with the butoxypropoxy chain 3HOmC1
exhibits the Cr-SmC*-N*-Iso phase sequence, similar to the
alkoxy compounds obtained by Inukai, while compound
3HO3C2 with the longer branch at the chiral centre has no
mesophase. Compound 3FO7C2 exhibits the widest tempera-
ture range of the synclinic, high tilted SmC* phase. The
members of the homologous series 3FOmC2, having an octan-3-
yloxy chiral moiety, exhibit much lower melting points than
compounds of the series 3FOmC1, having an octan-2-yloxy
chiral moiety. Changing the chiral part from oct-2-yloxy- to
oct-3-yloxy- decreases the melting as well as clearing points
signicantly. Replacing the partially uorinated butoxy unit in
the terminal achiral chain in compounds 3FOmCk by a butoxy
unit (compounds 3HOmCk) destabilizes the SmC* phase. The
comparison of the temperature range of the mesophases of
both series 3XOmC1 and 3XOmC2 is presented in Fig. 9. For
compounds 3HO3C1 and 3FO3C2 the temperature range of the
phase only during cooling are given.
Fig. 10 The structures of the series 3XOmC1 and 3XOOmC1; X ¼ F or
H.
The Goldstone mode is detected in the ferroelectric SmC*
phases. Fluorinated compounds have a higher dielectric
strength (d3) of the Goldstone mode than the protonated ones
and the relaxation frequency (fR) of the Goldstone mode is lower
for uorinated compounds than for the protonated ones. The
observed maximum optical tilt angle for uorinated
compounds is high: 44.9ꢀ for compound 3FO6C1 and 45.0ꢀ for
compound 3FO6C1. The observed maximum optical tilt angle
for protonated compound 3HO3C1 is a little bit lower, and is
about 42.9ꢀ. The values of the spontaneous polarization for all
Fig. 11 The comparison of the temperature ranges of phase
sequences for both series 3XOmC1 and 3XOOmC1.
phase sequence: Cr-SmC*-Iso. Compounds 3HO3C1 and
3HOO3C1 also show a similar phase sequence. The uorinated
compounds with an ester group in the achiral chain (nFOOmC1)
exhibit a low-tilted SmC* phase (the tilt angle value below 20ꢀ),
while uorinated compounds 3FOmC1 have a very high tilt
angle: 45ꢀ. The tilt angle of protonated compounds (nHOOmC1)
is quite high, for example: 40.3ꢀ at 60 ꢀC for compound
3HOO3C1, but lower than for compound 3HO3C1 (42.9ꢀ at 61
ꢀC). Both protonated families of compounds (3HO3C1 and
3HOO3C1) have an additional N* phase and a more ordered
chiral smectic phase below the SmC* phase. The replacement of
the partially uorinated terminal butoxy unit by the butoxy unit
in the achiral terminal chain led to the decrease of the stability
of the SmC* phase, similar to that observed for many other
liquid crystals. The length of the helical pitch in compounds
nFOOmC1 is similar to that in compounds 3FOmC1. The length
of the helical pitch in compounds nHOOmC1 is shorter than in
compounds 3HOmC1.
investigated compounds is relatively high, above 89.5 nC cmꢁ2
.
The length of the helical pitch decreases with the decrease of
the m index and is the shortest for compound 3FO3C1 (460.7
ꢀ
nm at 71 C).
An analogous series with an ester group in the terminal
achiral chain44,45 (marked as 3XOOmC1, see Fig. 10 and 11)
exhibits similar or slightly lower melting points and a similar
Acknowledgements
This work has been supported by the Polish Ministry of Science
and Higher Education, grant RMN No. 974/2014 and
POIG.01.03.01-14-016/08. We are thankful to Mr Edwin Myka for
his help in the preparation of compounds 3FOmC2.
Fig. 9 The comparison of the temperature ranges of phases in both
series 3XOmC1 and 3XOmC2.
81010 | RSC Adv., 2015, 5, 81003–81012
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